Osteoporosis and obesity are both important public health problems. Weight (wt) reduction is recommended to reduce obesity-related complications, but induces bone loss (even with adequate Ca intake) in post- menopausal women. Although several studies have begun to address the effects of diet composition during caloric restriction (CR) on metabolic parameters, virtually nothing is known its role on bone mass and quality. This proposal will address several major gaps in our knowledge concerning dietary composition during wt loss as it impacts bone health. In particular we will address dietary protein (pro) levels, and the role of vitamin D (vit D) in Ca absorption. Evidence in wt-stable adults shows that higher dietary pro increases insulin-like growth factor-1 and Ca absorption and may contribute to a higher bone mass and reduced fracture risk. In contrast, a high pro intake has also been shown to increase the acid load, leading to hypercalciuria, and potential bone loss. It is currently not known how a high pro diet affects bone density or quality during CR. In addition, the level of pro intake may be especially important during severe wt loss after gastric bypass surgery (GB), because of the deficient pro intake (rather than Ca and vit D) that typically follows the procedure that may exacerbate bone loss. Furthermore, because serum parathyroid hormone (PTH) explains part of the reduction in Ca absorption observed during CR (400IU vit D /day), it is possible that increasing serum 25-hydroxy-vit D levels through diet will attenuate serum PTH and enhance true fractional Ca absorption (TFCA). The aims of this proposal are: 1) to examine how a high-pro diet compared to a standard high-carbohydrate (HC) wt loss diet affects bone turnover, mass and quality in post- menopausal women, while controlling for normal Ca and vit D intake; 2) to examine bone with usual and higher pro intake during severe wt loss in women with GB; and 3) to determine if a high vit D intake (1200 lU/d) can increase TFCA in postmenopausal women during a standard HC wt-loss diet. Overall, rationale recommendations for the prevention of bone loss are needed for nearly % of the US population that is regularly dieting. The older population is going to double in only a few decades, and wt loss is a major determinant of bone loss, fracture risk, and ultimately mortality. Thus, the importance of these studies for identifying how diet composition can prevent bone loss in the older population is both critical and timely.